Recovery system using vehicle state information

ABSTRACT

A selection is made between single stage and multistage parachute deployment for a vehicle based at least in part on vehicle state information. In the event multistage parachute deployment is selected, a drogue parachute is deployed during a first deployment stage and afterwards, a main parachute is deployed during a second deployment stage. In the event single stage parachute deployment is selected, at least the main parachute is deployed in a single stage where in the event the drogue parachute is deployed during the single stage, the drogue and main parachute are deployed simultaneously.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/565,135 entitled RECOVERY SYSTEM USING VEHICLE STATEINFORMATION filed Sep. 9, 2019 which is incorporated herein by referencefor all purposes.

BACKGROUND OF THE INVENTION

New types of aircraft are being developed for usage in crowded urbanenvironments. For example, vertical takeoff and landing (VTOL) vehiclessuch as multicopters are attractive because for takeoff and landing theyonly require a small amount of space. This makes them feasible for usein more densely populated areas. However, one drawback of multicoptersis that they tend to be slow during forward flight. Traditional aircraftcan fly faster than multicopters during flight, but they require a longrunway for takeoff and landing. New types of vehicles, for example thatincorporate tiltrotors, are being developed to combine the small takeoffand landing footprint of multicopters with the faster forward flightspeeds of traditional aircraft. However, as these new types of vehiclesare developed, new types of problems not previously encountered bymulticopters or traditional aircraft can pop up. New types ofaccessories, components, and/or features which can accommodate these newtypes of vehicles would be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the followingdetailed description and the accompanying drawings.

FIG. 1 is a flowchart illustrating an embodiment of a process to deploya recovery system in either a single stage or multiple stages.

FIG. 2A is a diagram showing a perspective view of an embodiment of atiltrotor aircraft.

FIG. 2B is a diagram showing a front view of an embodiment of atiltrotor aircraft which performs a single stage parachute deploymentusing only the main parachute.

FIG. 2C is a diagram showing a front view of an embodiment of atiltrotor aircraft which performs a simultaneous parachute deployment.

FIG. 2D is a diagram showing a front view of an embodiment of atiltrotor aircraft which performs a multistage parachute deployment.

FIG. 3 is a table illustrating an embodiment where airspeed is used toselect between single stage parachute deployment and multistageparachute deployment.

FIG. 4A is a table illustrating an embodiment where the tilt angle ofthe tiltrotors is used to select between single stage parachutedeployment and multistage parachute deployment.

FIG. 4B is a diagram illustrating an example of a tiltrotor at a −90°tilt angle.

FIG. 4C is a diagram illustrating an example of a tiltrotor at a 0° tiltangle.

FIG. 5 is a flowchart illustrating an embodiment of a process to performa multistage parachute deployment where an amount of time between thedrogue parachute and the main parachute is determined based at least inpart on vehicle state information.

FIG. 6A is a graph illustrating an embodiment where ranges are used todetermine an amount of time between deployment of a drogue parachute anddeployment of a main parachute.

FIG. 6B is a graph illustrating an embodiment where a continuouslyvarying function is used to determine an amount of time betweendeployment of a drogue parachute and deployment of a main parachute.

FIG. 7A is a diagram illustrating an embodiment of a vehicle with avehicle velocity direction that is (substantially) straight down.

FIG. 7B is a diagram illustrating an embodiment of a vehicle with avehicle velocity direction that is at an angle.

FIG. 7C is a graph illustrating an embodiment of using vehicle velocitydirection to determine an amount of time between deployment of a drogueparachute and deployment of a main parachute.

FIG. 8 is a graph illustrating an embodiment of using acceleration todetermine an amount of time between deployment of a drogue parachute anddeployment of a main parachute.

FIG. 9A is a diagram illustrating an embodiment of a system diagramwhich includes redundant sensors, a flight controller, and a recoverysystem controller.

FIG. 9B is a diagram illustrating an embodiment of vehicle stateinformation signals that diverge over time.

FIG. 10 is a flowchart illustrating an embodiment of a process toperform a recovery process using saved vehicle state information that isknown to be good and recent.

FIG. 11 is a flowchart illustrating an embodiment of a process toperform a recovery process using saved vehicle state information that isknown to be good and recent.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as aprocess; an apparatus; a system; a composition of matter; a computerprogram product embodied on a computer readable storage medium; and/or aprocessor, such as a processor configured to execute instructions storedon and/or provided by a memory coupled to the processor. In thisspecification, these implementations, or any other form that theinvention may take, may be referred to as techniques. In general, theorder of the steps of disclosed processes may be altered within thescope of the invention. Unless stated otherwise, a component such as aprocessor or a memory described as being configured to perform a taskmay be implemented as a general component that is temporarily configuredto perform the task at a given time or a specific component that ismanufactured to perform the task. As used herein, the term ‘processor’refers to one or more devices, circuits, and/or processing coresconfigured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention isprovided below along with accompanying figures that illustrate theprinciples of the invention. The invention is described in connectionwith such embodiments, but the invention is not limited to anyembodiment. The scope of the invention is limited only by the claims andthe invention encompasses numerous alternatives, modifications andequivalents. Numerous specific details are set forth in the followingdescription in order to provide a thorough understanding of theinvention. These details are provided for the purpose of example and theinvention may be practiced according to the claims without some or allof these specific details. For the purpose of clarity, technicalmaterial that is known in the technical fields related to the inventionhas not been described in detail so that the invention is notunnecessarily obscured.

Various embodiments of a recovery system that makes decisions and/ordecides what type of parachute deployment strategy or mode to use aredescribed herein. In one example, the parachute deployment system isused in an aircraft with tiltrotors which enable the aircraft to flyover a much wider and/or different range of speeds compared to othertypes of aircraft (e.g., multicopters, traditional aircraft, etc.) thathave included recovery systems. In some embodiments, a recovery systemcontroller selects between single stage parachute deployment andmultistage parachute deployment for a vehicle based at least in part onvehicle state information associated with the vehicle. If multistageparachute deployment is selected (e.g., due to the vehicle's airspeed orsome other vehicle state information when the recovery process wasinitiated) then a drogue parachute is deployed during a first deploymentstage and after the drogue parachute has deployed, a main parachute isdeployed during a second deployment stage. Otherwise, if single stageparachute deployment is selected, at least the main parachute isdeployed in a single stage, wherein in the event the drogue parachute isdeployed during the single stage, the drogue parachute and the mainparachute are deployed simultaneously. For single stage, in someembodiments, only the main parachute is deployed. In some other singlestage embodiments, both the drogue parachute and the main parachute aredeployed (simultaneously). In some embodiments, an airspeed threshold isused to decide whether to only deploy the main parachute or both thedrogue parachute and main parachute simultaneously during single stagemode. For example, both parachutes are deployed simultaneously duringsingle stage mode if the vehicle's airspeed is above some threshold andonly the main parachute is deployed is below the threshold.

FIG. 1 is a flowchart illustrating an embodiment of a process to deploya recovery system in either a single stage or multiple stages. In someembodiments, the process is performed by a controller in a recoverysystem, sometimes referred to herein as a recovery system controller.For example, the recovery system may include (in addition to therecovery system controller) a main parachute, which is always deployed,and a drogue parachute, which is (optionally) not deployed (e.g., insome single stage deployment embodiments), deployed at the same time asthe main parachute (e.g., in some single stage deployment embodiments),or deployed before the main parachute, The process shown here isperformed or otherwise initiated when there is a decision to use orotherwise deploy the recovery system (i.e., deploy the parachute(s)). Insome cases, a pilot (e.g., manually) initiates this process and then therecovery system controller uses the process shown here to decide whichparachute deployment mode to use. Or, the recovery system controller may(e.g., automatically, without guidance or instruction from the pilot)detect or otherwise determine that there is some (catastrophic) failurewhich requires the deployment of the recovery system.

At 100, a selection is made between single stage parachute deploymentand multistage parachute deployment for a vehicle based at least in parton vehicle state information associated with the vehicle. In someembodiments, the vehicle state information that is used to make thisselection is the vehicle's airspeed. In some applications, the vehicleincludes tiltrotors which (when pointing in the appropriate direction orangle) enable the vehicle to hover (e.g., at airspeeds at or near 0) orperform wing-borne flight (e.g., at significantly higher airspeeds). Insome embodiments, additional and/or other types of vehicle stateinformation (e.g., besides airspeed) is/are used (e.g., the tilt angleof the tiltrotors, etc.).

In some embodiments, a vehicle's altitude is not used directly indeciding whether to perform single stage parachute deployment ormultistage parachute deployment at step 100, but the vehicle's altitudeis used by the flight computer to constrain the vehicle at certainaltitudes to certain airspeeds which in turn causes certain (e.g.,better) parachute deployment mode selections or decision. For example,below some altitude threshold, the flight computer may limit thevehicle's (maximum) airspeed and the lower and/or limited airspeedsensure or otherwise guarantee selection of single-stage deployment modeat these lower altitudes. Above that altitude threshold, the pilot ispermitted by the flight computer to fly as fast as desired and dependingupon the pilot's selected flight speed, either multistage or singlestage deployment mode is selected. In some applications, having theflight computer control or otherwise limit the vehicle's airspeed inthis manner based on the vehicle's altitude is desirable because itallows the system to have maximum coverage and to minimize the minimumaltitude required for full parachute deployment and deceleration beforetouchdown. In particular, activating the multistage deployment modebelow a certain altitude could result in insufficient time for theparachute(s) to sufficiently inflate and slow the vehicle down beforeimpact, which is undesirable. In this manner, the flight controller mayoperate in manner that helps the recovery system controller to make abetter selection about how to deploy the parachute(s). Having the flightcomputer limit the vehicle's (maximum) airspeed below some altitudethreshold may also be desirable because a vehicle is more likely toencounter obstacles (e.g., trees, building, electrical or telephonewires, etc.) at lower altitudes and flying at slower airspeeds may givethe pilot more time to detect and avoid collisions or at least reducedamage if there is a collision.

In various embodiments, a variety of sensors are used to measure thevehicle state information used at step 100. In one example, airspeed isthe vehicle state information used at step 100 and the airspeed ismeasured using one or more pitot tubes. The measured airspeed may besent from a pitot tube to a flight controller (sometimes referred toherein as a flight computer) and the flight controller may forward theairspeed on to a recovery system controller (e.g., that controls therecovery system, such as when it is appropriate to automatically deploythe recovery system, whether to deploy the drogue parachute first ornot, etc.). In another example, if the vehicle state information that isused is the tilt angle of a vehicle's tiltrotors, then a servo sensor(e.g., associated with and/or which receives information from a servowhich controls the position of the tiltrotors) may be used. In someembodiments, acceleration is used and an inertial measurement unit (IMU)is used to measure the vehicle's acceleration. Any appropriate sensormay be used based on the particular application (e.g., what is alreadyavailable in the vehicle, taking into account any space or weightlimitation(s), etc.).

In some embodiments, vehicle state information is filtered, smoothed,and/or sampled (e.g., selectively) before being used to make a selectionat step 100. For example, if a major structural failure is detected,then the pitot tube readings (i.e., vehicle airspeed) are assumed to bebad due to incorrect vehicle orientation past the point in time of thefailure. Filtering or otherwise processing recent data as describedabove (e.g., ignoring the data once or if a failure is detected) helpsto ensure that the correct mode is selected. Sensor information ofvarious types from multiple channels or sources (e.g., pitot, tilt, IMU)may also be combined and/or weighted together in order to improve theaccuracy of mode selection (e.g., as more is learned about optimalparachute deployment).

Some examples of pieces or types of vehicle state information that canbe used to select parachute deployment mode (e.g., at step 100) and/or(as will be described in more detail below) determine an amount of delaybetween a drogue parachute and a main parachute include: vehicleairspeed, vehicle tilt angle, direction of vehicle velocity (e.g., froman IMU, covered in detail), acceleration (from an IMU, accelerometers,etc.), angle of attack, vehicle orientation, and/or vehicle rates (e.g.,from a gyroscope).

In the event multistage parachute deployment is selected (102), a drogueparachute is deployed during a first deployment stage at 104. At 106,after the drogue parachute has deployed, a main parachute is deployedduring a second deployment stage. For example, if the vehicle is flyingat a relatively high speed (e.g., above some airspeed threshold), thedrogue parachute will slow the vehicle down until it is flying at asufficiently slow speed where the main parachute can be safely deployedwithout the main parachute tearing or ripping. The main parachute thenfurther slows down the vehicle's descent.

In various embodiments, the amount of time between deployment of thedrogue parachute (at step 104) and the main parachute (at step 106) maybe determined in a variety of ways. In some embodiments, a predefined orfixed lag or amount of time between the two is used. In someembodiments, the amount of time is determined based on (current) vehiclestate information. The vehicle state information used to calculate thelag may be the same set of state information used at step 100 to selectthe parachute deployment type, or may be a different set of stateinformation.

Returning to step 102, in the event the selected parachute deploymenttype is single stage, at 108 at least the main parachute is deployed ina single stage, wherein in the event the drogue parachute is deployedduring the single stage, the drogue parachute and the main parachute aredeployed simultaneously. For example, both the drogue parachute and themain parachute may be deployed simultaneously at step 108. Or, the mainparachute may be deployed by itself and the drogue parachute is notdeployed at all. If the aircraft is flying at a low rate of speed (see,e.g., the example ranges discussed below with respect to FIG. 3) then itis not necessary to perform multistage deployment where first the drogueparachute is deployed and then subsequently the main parachute isdeployed with some lag or delay between the two. In fact, deploying adrogue parachute and then subsequently the main parachute when it is notnecessary to do so can be detrimental if the aircraft is at a relativelylow altitude. In such cases, the aircraft will need as much time aspossible with the main parachute inflated in order to slow down theaircraft to a safe (crash) speed. As such, in one example, if theairspeed is below some airspeed threshold then single stage deploymentis performed where (1) only the main parachute is deployed or (2) thedrogue and the main parachutes are deployed simultaneously, becausedeploying the drogue parachute and then subsequently the main parachute(i.e., performing multistage deployment) would unnecessarily delaydeployment of the main parachute and thus unnecessarily delaydeceleration of the vehicle.

In one example application, the process of FIG. 1 is used in an aircraftwith tiltrotors which enable the aircraft to perform vertical takeoffsand/or vertical landings. The following figures show an exampletiltrotor aircraft which may perform the exemplary process shown in FIG.1.

FIG. 2A is a diagram showing a perspective view of an embodiment of atiltrotor aircraft. In the example shown, the aircraft (200 a) includesa canard (202) and a main wing (204). In this aircraft, there are twotiltrotors (206 a) attached to the canard and six tiltrotors (208 a)attached to the main wing. The tiltrotors in this example are attachedto the canard and main wing via bollards which include a tiltingmechanism (not shown) which permits the tiltrotors to point downward(shown in the next figure).

When the tiltrotors (206 a and 208 a) are pointing down (not shownhere), the downward thrust from the tiltrotors provides sufficient liftto keep the vehicle airborne. In other words, the aircraft is able tohover mid-air and fly at relatively low speeds (e.g., at or near 0knots). If desired, the vehicle can perform vertical takeoffs and/orvertical landings by having the tiltrotors point down.

If the tiltrotors (206 a and 208 a) are rotated to point backwards(shown here), the aircraft transitions to a forward flight mode wherethe aircraft is moving forwards fast enough so that the aircraft is keptairborne by the aerodynamic lift force on the main wing (204), sometimesreferred to herein as wing-borne flight. In this mode, the aircraft isable to fly at relatively high speeds (e.g., a maximum speed of 150knots during normal flight). By making both single stage and multistageparachute deployment available and selecting the appropriate one in realtime using vehicle state information associated with the vehicle, therecovery system can be used over the entire range of speeds for theexemplary aircraft (e.g., anywhere from 0 knots to 150 knots).

In contrast, a traditional fixed-wing, fixed-rotor aircraft (e.g., whichalways performs wing-borne flight) cannot hover mid-air and does notneed to worry about deploying a parachute at or near 0 knots per hour.Such a traditional aircraft has a different range of speeds andtherefore its recovery system would not have to encounter the same setof flight and parachute deployment conditions.

The example aircraft shown here is also different from a multicopter(e.g., with rotors in fixed positions that generally point downwards)because such multicopters cannot fly in a forward flight or wing-borneflight mode. As such, multicopters tend to be much slower than tiltrotorvehicles (such as the one shown here).

This means that recovery systems which were developed for traditionalfixed-wing, fixed-rotor aircraft or for multicopters will notnecessarily work with tiltrotor aircraft because the range of speedsover which a tiltrotor aircraft can fly is quite different from therange of speeds of a traditional aircraft or multicopter.

The recovery system embodiments disclosed herein have many advantagesover conventional systems. In one aspect, the recovery systemembodiments minimize altitude loss including for tiltrotor aircraft. Forexample, with a one-size-fits-all approach where the same deploymentsequence is performed (e.g., always perform multistage deployment), theaircraft could unnecessarily lose altitude. The tiltrotor aircraft mayfly at lower altitudes than other types of aircraft and so anyunnecessary altitude loss is dangerous for the pilot/passengers. Inanother aspect, the recovery system embodiments reduce loading on anoccupant or vehicle at attachment points. For example, if single stagedeployment were always performed, this could put dangerous load levelson the occupant or vehicle, causing harm to the occupant, causingstructural overload of the airframe or parachute, and/or resulting inunnecessarily high system mass to account for high loads.

FIG. 2B is a diagram showing a front view of an embodiment of atiltrotor aircraft which performs a single stage parachute deploymentusing only the main parachute. In this example, the aircraft (210) isshown from the front with the rotors (206 b and 208 b) on and pointingand thrusting (generally) downwards. For example, when the tiltrotorsare angled downward as shown here, the vehicle may be hovering mid-airor flying at a relatively low speed. In this mode (e.g., at lower speedsand/or when the tiltrotors are angled in a generally downwardsdirection), one deployment strategy is to perform single stage parachutedeployment where only the main parachute (220 b) is deployed and thedrogue parachute (not shown) is not deployed, as shown here. This is oneexample of how step 108 in FIG. 1 may be performed.

Alternatively, in some embodiments, both parachutes are deployedsimultaneously in this mode (e.g., at lower speeds and/or when thetiltrotors are pointing downwards). The following figure shows anexample of this.

FIG. 2C is a diagram showing a front view of an embodiment of atiltrotor aircraft which performs a simultaneous parachute deployment.As in the previous example, the rotors (206 c and 208 c) are pointingand thrusting downwards so that the vehicle is flying at a relativelylow speed (e.g., hovering mid-air or flying forwards slowly). In thisexample, the drogue parachute (230 c) and main parachute (220 c) aredeployed simultaneously. In some embodiments, the drogue parachute iscut away or otherwise detached from the main parachute after bothparachutes are fully inflated (at least for some simultaneous deploymentembodiments). If the two parachutes were deployed separately with somedelay between the two in the state shown here, that delay wouldunnecessarily delay deployment of the main parachute and the vehiclewould unnecessarily lose altitude before the main parachute fullyinflated. This is another example of how step 108 in FIG. 1 may beperformed.

In this example, to further reduce inflation time, the main parachute(220 c) and drogue parachute (230 c) are ballistic parachutes which aredeployed using one or more rockets (222 c).

FIG. 2D is a diagram showing a front view of an embodiment of atiltrotor aircraft which performs a multistage parachute deployment. Inthis example, the rotors (206 d and 208 d) are pointing and thrusting(generally) backwards. In this mode (e.g., at higher speeds and/or whenthe tiltrotors are angled in a generally backwards direction), amultistage parachute deployment is performed in this example where thedrogue parachute (230 d) is first deployed, and then the main parachute(220 d) is deployed. In some embodiments, the drogue parachute is cutaway or otherwise detached from the main parachute at some point (e.g.,after the main parachute inflates). As with the previous examples, inthis example, the main parachute is a ballistic parachute where one ormore rockets (222 d) are used to extract and inflate the parachute.

In some embodiments, if both airspeed and tilt angle are available to arecovery system controller, the airspeed is used to select betweensingle stage and multistage parachute deployment because the results arebetter and/or more accurate. Naturally, in some embodiments tilt anglemay be used instead of airspeed (e.g., because the airspeed informationis compromised and/or potentially bad but the tilt angle information isnot).

The following figures describe more specific examples of vehicle stateinformation which may be used to select between single stage parachutedeployment versus multistage parachute deployment.

FIG. 3 is a table illustrating an embodiment where airspeed is used toselect between single stage parachute deployment and multistageparachute deployment. In this example, the (measured) airspeed iscompared against an airspeed threshold (airspeed_thresh) in order toselect the parachute deployment mode or type. Column 300 shows tworanges of airspeeds and column 302 shows the corresponding type ofparachute deployment that is selected. If the (measured) airspeed isbetween 0 and some airspeed threshold (e.g., inclusive), then singlestage parachute deployment is selected per row 304. If the (measured)airspeed is (e.g., strictly) greater than the airspeed threshold andless than some maximum airspeed (airspeed_max), then multistageparachute deployment is selected (see row 306). As a practical matter,it may not be necessary to use or know the maximum airspeed In oneexample, an airspeed threshold of 30 meters per second is used for theexemplary aircraft described above which can fly at speeds between 0knots and 150 knots (˜87.45 meters per second).

FIG. 4A is a table illustrating an embodiment where the tilt angle ofthe tiltrotors is used to select between single stage parachutedeployment and multistage parachute deployment. In this example, the(measured) tilt angle is compared against a tilt angle threshold(tilt_angle_thresh) in order to select the parachute deployment mode ortype. For simplicity and ease of explanation, it is assumed that thetiltrotors have a 90° range of movement. Column 400 shows two ranges oftilt angles and column 402 shows the corresponding parachute deploymenttype or mode that is selected. In this example, if the tilt angle isbetween −90° and the tilt angle threshold (e.g., −60°), then singlestage parachute deployment is selected (see row 404). If the tilt angleis between the tilt angle threshold and 0°, then multistage parachutedeployment is selected (see row 406).

The following figures illustrate an example of the orientation oftiltrotors at −90° and 0° tilt angles, respectively.

FIG. 4B is a diagram illustrating an example of a tiltrotor at a −90°tilt angle. In this example, tiltrotor 410 is pointing downwards and hasa tilt angle of −90°. See also tiltrotors 206 b and 208 b in FIG. 2B or206 c and 208 c in FIG. 2C.

FIG. 4C is a diagram illustrating an example of a tiltrotor at a 0° tiltangle. In this example, tiltrotor 420 is pointing backwards and has atilt angle of 0°. See also tiltrotors 206 c and 208 c in FIG. 2B or 206c and 208 c in FIG. 2C.

In some embodiments, a flight controller maintains a state variable thatit uses to keep track of a vehicle's mode of flight. For example, forthe vehicle shown in FIGS. 2A-2D, the vehicle mode of flight (i.e.,values of the state variable) can be: hover, forward flight, ortransition(al) (e.g., the vehicle is transitioning from hover to forwardflight or vice versa). In some embodiments, a vehicle's mode of flight(e.g., a state variable which is managed by the flight controller) isused to select between single stage parachute deployment and multistage.For example, if the mode is hover, then single stage deployment isselected. If the mode is forward flight, then multistage deployment isselected. If the mode is transition then single-stage deployment isselected if airspeed is less than the airspeed threshold, and multistagedeployment is selected if airspeed is greater than the airspeedthreshold. If vehicle operations guarantee that airspeed is always belowor above the airspeed threshold in transition mode, then single-stage ormultistage mode may be selected, respectively, without referencingairspeed.

Returning briefly to FIG. 1, in some embodiments, a fixed amount of timeis used for the time between deployment of the drogue parachute at step104 and deployment of the main parachute at step 106. For example, theworst case scenario (i.e., longest amount of time) could be calculatedand that value could be used anytime (e.g., regardless of what thevehicle's sensors are reporting about the vehicle's state) a multistageparachute deployment is performed. Alternatively, a (real-time)determination or selection of the amount of time between the drogueparachute and main parachute is made in some embodiments. The followingfigures describe some examples of this.

FIG. 5 is a flowchart illustrating an embodiment of a process to performa multistage parachute deployment where an amount of time between thedrogue parachute and the main parachute is determined based at least inpart on vehicle state information. Although not explicitly described inthis example, in some embodiments the drogue parachute is jettisoned(e.g., cut or otherwise decoupled from the main parachute and/or therest of the system) after the main parachute is deployed. Forconvenience and ease of explanation, the same or similar referencenumbers are used to indicate related steps from FIG. 1.

At 500, an amount of time between deployment of a drogue parachute anddeployment of a main parachute is determined based at least in part onvehicle state information associated with the vehicle. In someembodiments, the same vehicle state information that is used to selectbetween single stage parachute deployment and multistage parachutedeployment (e.g., at step 100 in FIG. 1) is used at step 500 todetermine the amount of time. In one example, airspeed is used to selectbetween single stage and multistage parachute deployment and (assumingmultistage is selected) the airspeed is also used to determine the lagor delay between the drogue parachute and the main parachute. Someexamples of this are described in more detail below.

Alternatively, in some embodiments, a second set of vehicle stateinformation is used (e.g., at step 500) that is different from thevehicle state information that is used to select between single stageand multistage parachute deployment (e.g., at step 100 in FIG. 1). Inone example, there is an additional piece of information used at step500 that is not used at step 100 in FIG. 1. Some examples of this aredescribed in more detail below.

At 104, the drogue parachute is deployed. At 106′, after the drogueparachute has deployed and the determined amount of time has elapsed,the main parachute is deployed. For example, the drogue parachute (230d) in FIG. 2D is deployed first to slow down the aircraft before themain parachute (220 d) is deployed. If the amount of time determined atstep 500 is T_lag, then in FIG. 2D the main parachute (220 d) would bedeployed T_lag after deployment of the drogue parachute (230 d).

The following figures describe more detailed examples of vehicle stateinformation that may be used to calculate, select, or otherwisedetermine an amount of time between deployment of a drogue parachute anda main parachute.

FIG. 6A is a graph illustrating an embodiment where ranges are used todetermine an amount of time between deployment of a drogue parachute anddeployment of a main parachute. In this example, a single piece or typeof vehicle state information is used to determine an amount of timebetween deployment of a drogue parachute and deployment of a mainparachute (T_lag) and that single piece or type of vehicle stateinformation used to determine T_lag is the same one used to selectbetween single stage and multistage parachute deployment. Although threebins or ranges are shown in this example, a different number of bins orranges may be used if desired.

In this example, the x-axis of the graph shows the vehicle stateinformation values and the y-axis shows the corresponding T_lag value(i.e., the amount of time between deployment of a drogue parachute anddeployment of a main parachute). In some embodiments, the vehicle stateinformation used to determine T_lag is airspeed (e.g., T_lag=f(AS)). Insome embodiments, the vehicle state information used is the tilt angleof the tiltrotors (e.g., T_lag=f(TA)).

In this example, bins or ranges are defined where if the value of thevehicle state information falls into a particular bin, a correspondingT_lag value is selected. For example, if the value of the airspeed isbetween airspeed_thresh (600) and airspeed_0 (602), then a delay or lagof T0 (604) is selected for T_lag. In this example, it is assumed thatairspeed_thresh, or alternatively tilt_angle_thresh, (600) is a cutoffabove which multistage parachute deployment is performed and below whichsingle stage parachute deployment is performed. Similarly, if the tiltangle is used then a T_lag value of T0 (604) is selected for tilt anglesbetween tilt_angle_thresh (600) and tilt_angle_0 (602).

For the higher airspeeds or higher tilt angles (e.g., a more level orhorizontal position) corresponding to the next bin, a larger T_lag valueof T1 (606) is selected for values between airspeed_0/tilt_angle_0 (602)and airspeed_1/tilt_angle_1 (608). The largest T_lag value of T2 (610)is selected for values above airspeed_1/tilt_angle_1 (608).Conceptually, if the vehicle is flying faster (e.g., indicated by ahigher airspeed or a tilt angle which is closer to 0°), then more timewill be required for the drogue parachute to sufficiently slow down thevehicle before the main parachute can be safely deployed.

For completeness it is noted that the bin or range boundaries (e.g.,airspeed_thresh/tilt_angle_thresh, airspeed_0/tilt_angle_0, etc.) may behandled or mapped to a particular T_lag value in any appropriate and/ordesired manner. For example, the values for which a T_lag of T0 isselected may be [airspeed_thresh, airspeed_0] or [tilt_angle_thresh,tilt_angle_0]. This applies to other examples described herein.

Alternatively, in some embodiments a function used to determine T_lag isa continuously varying function. The following figure shows an exampleof this.

FIG. 6B is a graph illustrating an embodiment where a continuouslyvarying function is used to determine an amount of time betweendeployment of a drogue parachute and deployment of a main parachute. Aswith the previous example, the vehicle state information used to selectbetween single mode and multimode parachute deployment is the same asthe vehicle state information that is used to determine T_lag. Thex-axis of the graph shows the vehicle state information values (e.g.,airspeed or tilt angle) and the y-axis shows the corresponding T_lagvalue.

In this example, a linear function (620) is used to determine orotherwise calculate T_lag based on the value of the vehicle stateinformation (e.g., airspeed or tilt angle). At a cutoff value (622),such as airspeed_cutoff or tilt_angle_cutoff, the T_lag value is Tmin(624) T_lag increases linearly from there. In some embodiments, therange of T_lag values includes zero, which corresponds to a single-stagesimultaneous deployment as described above (see, e.g., drogue parachute230 c and main parachute 220 c in FIG. 2C). For example, the T_lagfunction may be a sliding scale ranging from simultaneous deploy (i.e.,Tmin=0) up to some maximum lag between the two deployments as airspeedincreases. The linear function (620) shown here is merely exemplary andany appropriate function may be used. In some embodiments, the functionis implemented as a lookup table for ease of implementation and/oraccess speed.

In the examples described above, a single-input function is used tocalculate or otherwise determine T_lag. In some embodiments, two or moreinputs (e.g., two different types of vehicle state information) are usedto determine T_lag. The following figures describe an example of this.

FIG. 7A is a diagram illustrating an embodiment of a vehicle with avehicle velocity direction that is (substantially) straight down. Inthis example, the tiltrotors (700) of the vehicle are pointing straightdown and the vehicle is hovering at or near an airspeed of 0 before somecatastrophic failure occurred which caused the recovery system to bedeployed (e.g., either manually by the pilot or automatically by therecovery system controller). As a result of the vehicle's low airspeedat the time of the catastrophic failure, the direction of the vehicle'svelocity (702), sometimes referred to herein as the vehicle velocitydirection, is pointing straight down (e.g., at an angle of −90° relativeto a horizontal axis). When a vehicle is falling or otherwise movingstraight down with some starting velocity (as shown here), the peak netairspeed during the opening sequence will be greater than the case inwhich the vehicle is moving forward with the same starting velocity. Asa result, knowing initial direction of vehicle velocity relative togravity can help determine peak airspeed during the opening sequence andassist generally in mode selection and optimization of time delays. Ingeneral, the drogue parachute (should) stay open longer the more thevehicle velocity direction points straight down, or aligns with gravity,to facilitate vehicle recovery.

FIG. 7B is a diagram illustrating an embodiment of a vehicle with avehicle velocity direction that is at an angle. In this example, thetiltrotors (710) of the vehicle are pointing straight back and thevehicle was flying forwards at (generally speaking) a moderate to highrate of speed. As such, if a (catastrophic) failure were to occur inthis state, the vehicle velocity direction (712) will be at an angle(e.g., −15° relative to a horizontal axis). With the drogue parachutedeployed in this state (i.e., with this vehicle velocity direction), thevehicle peak airspeed will be lower (e.g., for a same or given startingairspeed) compared to the vehicle shown in FIG. 7A. In other words, ingeneral, the closer the vehicle velocity direction is to a horizontalaxis, the lower the vehicle peak net airspeed during the deploymentsequence. This allows higher starting airspeeds for single-stage mode orshorter delays at angles approaching horizontal, assuming a fixed peaktotal airspeed that is allowed before occupant, vehicle, or parachuteload limits are reached.

In some embodiments, vehicle velocity direction is used in addition tosome other vehicle state information (e.g., airspeed) to determine anamount of time between deployment of a drogue parachute and a mainparachute (T_lag). The following figure shows an example of this.

FIG. 7C is a graph illustrating an embodiment of using vehicle velocitydirection to determine an amount of time between deployment of a drogueparachute and deployment of a main parachute. In this example, twoinputs or pieces of vehicle state information are used to determineT_lag, one of which is vehicle velocity direction (i.e., T_lag=f(·,VVD)). For simplicity and ease of explanation, the other pieces ofvehicle state information (e.g., airspeed or tilt angle) are fixed inthis graph (i.e., a 2D graph is shown here to illustrate characteristicsof the more complete 3D graph). In the graph shown here, the x-axisshows vehicle velocity direction (e.g., ranging from −90° to 0°) and they-axis shows the corresponding T_lag value.

In this example, T_lag is a linear function (720) of the vehiclevelocity direction where the other input (e.g., airspeed or tilt angle)is fixed. When the vehicle velocity direction is −90° (722), T_lag valueof T4 (724) is generated or otherwise output. For a vehicle velocitydirection of 0° (726), the T_lag value is T3 (728) where T3<T4. Althoughthe example function shown here is linear, any type of function may beused.

As an example of what a corresponding 3D graph would look like (e.g., ifthe other piece of vehicle state information, such as airspeed or tiltangle, were permitted to vary), it is noted that T_lag generallyincreases as the airspeed or tilt angle increases, as shown in FIG. 6Aand FIG. 6B. Therefore, the largest T_lag value will (generallyspeaking) be generated when the vehicle velocity direction is at or near−90° (i.e., the vehicle velocity direction is straight down or close toit) and the airspeed or tilt angle is at or near its maximum value.Conversely, the smallest T_lag value will (generally speaking) begenerated when the vehicle velocity direction is at or near 0° (i.e.,the vehicle velocity direction is horizontal or close to it) and theairspeed or tilt angle is at or near 0. In some embodiments, modelingand/or testing may be used to interpolate and/or otherwise determinesome or all of the T_lag values generated by f(·, VVD) or whateverfunction is being used.

In the example of FIG. 7A-FIG. 7C, the vehicle state information used togenerate the amount of time between the drogue parachute and the mainparachute (i.e., T_lag) is different from the vehicle state informationthat is used to select between single stage and multistage parachutedeployment. For example, just the airspeed or just the tilt angle may beused to select between the types of parachute deployment, while theairspeed and the vehicle velocity direction (or alternatively the tiltangle and the vehicle velocity direction) may be used to generate thevalue of T_lag. The following figure describes another example where thetwo sets of vehicle state information are different from each other (butas described herein in some embodiments the same metrics or vehiclestate information are used for mode selection and delay determination).

FIG. 8 is a graph illustrating an embodiment of using acceleration todetermine an amount of time between deployment of a drogue parachute anddeployment of a main parachute. In this example, the acceleration of thevehicle is analyzed over time after the drogue parachute deploys (800).Conceptually, the acceleration is examined to identify when it hasleveled out. This is an indication that the drogue parachute has slowedthe vehicle down to the extent possible and that it is time to deploythe main parachute. In this example, when ΔA/Δt (alternatively, thederivative of the acceleration function, dA/dt) drops below somethreshold (e.g., close to 0), a decision is made to deploy the mainparachute (802).

As described above, this is one example where the vehicle stateinformation used to determine T_lag is different from the vehicle stateinformation that is used to select between single stage and multistageparachute deployment. For example, the vehicle's acceleration may beused for the former and the vehicle's airspeed (or, alternatively, tiltangle) may be used for the latter.

In some embodiments, a given type of vehicle state information (used forwhatever purpose) is reported from multiple sensors. The followingfigures describe some examples where the sensors report different and/orconflicting vehicle state information and steps that may be taken inresponse.

FIG. 9A is a diagram illustrating an embodiment of a system diagramwhich includes redundant sensors, a flight controller, and a recoverysystem controller. In the example shown, three independent and redundantsensors (900 a-900 c) are used to report a given type of vehicle stateinformation. For example, if airspeed is being reported, all of thesensors might be pitot tubes which report independent airspeed values.The sensors pass on their measured values (i.e., vehicle stateinformation) to the flight controller (902). It is noted that the numberof sensors shown herein is merely exemplary and any number of sensorsmay be used.

In this example, the recovery system controller (904) periodically sendsa status query to the flight controller (902). The flight controllerresponds with a reply. In some embodiments, the reply includes thestatus of the flight controller and/or the status of the sensors. Forexample, if the flight controller becomes aware it is compromised but isstill able to communicate with the recovery system controller, the replymay include “flight_computer_health=BAD” (as an example). In someembodiments, vehicle state information from the sensors is included inthe reply (e.g., on a regular basis, even if the flight controllerdetects no failures in the sensors or the vehicle overall).

As will be described in more detail below, including vehicle stateinformation (which is agreed upon by at least 2 out of 3 sensors) mayenable the recovery system controller to have a copy of vehicle stateinformation that is known to be good and recent (fresh) in case theflight controller and/or sensors fail and the recovery system controllerneeds good and recent vehicle state information to (as an example)select between single stage and multistage parachute deployment and/ordetermine an amount of time between a drogue and main parachute.

FIG. 9B is a diagram illustrating an embodiment of vehicle stateinformation signals that diverge over time. In this example, the threevehicle state information signals (920 a-920 c) are respectively outputby sensors 900 a-900 c from FIG. 9A. Initially, all vehicle stateinformation signals (920 a-920 c) agree at time 922.

In this state, flight controller 902 in FIG. 9A (assuming it isfunctional) might (as an example) send to recovery system controller 904a reply of “flight_computer_health=GOOD; VSI_0=COMMON_VALUE0;VSI_1=COMMON_VALUE0; VSI_2=COMMON_VALUE0”. In this example, all threevehicle state information signals are included in the reply whichpermits the recovery system controller (904 in FIG. 9A) to know that allthree are in agreement with each other. Naturally, this information maybe conveyed between the flight controller and recovery system in avariety of ways and these message fields and/or format is merelyexemplary and is not intended to be limiting.

Later, at time 924, VSI 0 (920 a) diverges from the other vehicle stateinformation signals (920 b and 920 c). In other words, 2 out of 3vehicle state information signals agree with each, down from 3 out of 3.

In this state, flight controller 902 in FIG. 9A might send a reply torecovery system controller 904 with “flight_computer_health=GOOD;VSI_0=VALUE A; VSI_1=COMMON_VALUE1; VSI_2=COMMON_VALUE1”. In thisexample, the flight controller (e.g., 902 in FIG. 9A) will force thevehicle to land as soon as possible when one of the sensors reportsdivergent vehicle state information from the other sensors.

While the vehicle is descending but before the vehicle has landed, allthree vehicle state information signals (920 a-920 c) disagree with eachother at time 926. For example, flight controller 902 in FIG. 9A wouldsend to recovery system controller 904 a reply of“flight_computer_health=GOOD; VSI_0=VALUE_B; VSI_1=VALUE_C;VSI_2=VALUE_D”. In response, the recovery system controller 904 may(automatically) decide to deploy the recovery system.

Before any of the parachutes are actually launched, the recovery systemcontroller selects between single stage and multistage parachutedeployment (e.g., per FIG. 1). To make that decision, the recoverysystem controller will use the most recent vehicle state informationwhere at least 2 of the 3 signals agreed with each other. In thisexample, the values reported by VSI 1 (920 b) and VSI 2 (920 c) at time924 are used to make any decisions and/or calculations since thosesignals are the last time that at least two of the signals are inagreement with each other and they are therefore known to be recent andgood. This information is deemed to be both good (e.g., because it isunlikely that this information is inaccurate) and recent (e.g., becauseat most this information is from the second-most-recent reply). It wouldbe bad, for example, to make decisions about the parachute using badand/or old vehicle state information. Similarly, if the recovery systemcontroller is configured to determine T_lag (i.e., this is not anembodiment where a fixed T_lag value is used), the most recent vehiclestate information when at least 2 of the 3 signals agreed with eachother would be used.

The following figures describe this more formally and/or generally inflowcharts.

FIG. 10 is a flowchart illustrating an embodiment of a process toperform a recovery process using saved vehicle state information that isknown to be good and recent. In this example, there are assumed to bethree independent signals or sensors but naturally any number may beused. In some embodiments, this process is used in combination with FIG.1.

At 1000, three or more independent signals are monitored, each of whichindependently reports the vehicle state information. For example, inFIG. 9A, the recovery system controller (904) gets the three independentsignals from the reply message from the flight controller (902).Alternatively, the recovery system controller (904) may have a directpath to the sensors (900 a-900 c) that bypasses the flight controller(902).

At 1002, it is determined if a majority of the three independent signalsagree. For example, in FIG. 9B, the decisions would be Yes at time 922(because all three signals 920 a-920 c agree at that time), Yes at time924 (because signals 920 b and 920 c agree at that time), and No at time926 (because all three signals disagree at that time).

If it is determined at 1002 that a majority agree, then the vehiclestate information from the majority of the independent signals thatagree is saved at 1004. For example, even if three out of three signalsare reporting the same information and the flight controller isdetecting no issues, it would be better to save the information since afailure may occur suddenly and unexpectedly.

After saving the vehicle state information at step 1004, it isdetermined at 1006 whether to end the process. For example, this processmay run continuously until the vehicle lands. If it is decided not toend the process at 1006 then the independent signals continue to bemonitored at step 1000.

Returning to step 1002, if it is determined that a majority of theindependent signals do not agree, then at 1008 a recovery process isperformed using the most recently saved vehicle state information. Forexample, a recovery system controller may perform the process of FIG. 1using the most recently saved information (e.g., saved during the lastiteration through step 1004) which is known to be good and recent. Or,in FIG. 5, the most recently saved vehicle state information may be usedto determine an amount of time between deployment of a drogue parachuteand a main parachute.

The technique described above (where vehicle state information is savedfor use if needed) may be attractive in applications where there is avery strict vehicle weight limit and/or the vehicle is relatively costconstrained because new and/or additional sensors are not required. Analternative solution, for example, would be to have the recovery systemcontroller have its own sensor(s) that is/are independent from thesensors (e.g., 900 a-900 c) that report to the flight controller. Forvehicles with more lenient weight and cost restrictions, this may be anattractive solution.

Returning briefly to FIG. 9A, in some cases, the flight controller (902)may not respond at all to the query from the recovery system controller(904), or the flight controller may indicate that it has detected someerror or failure in itself and/or other components or devices in thevehicle. The following figure describes an example where a recoveryprocess is automatically initiated by a recovery system controller(e.g., in response to a lack of response from the flight controllerand/or an error or failure reported by the flight controller).

FIG. 11 is a flowchart illustrating an embodiment of a process toperform a recovery process using saved vehicle state information that isknown to be good and recent. In one example, the process is performed byrecovery system controller 904 in FIG. 9A.

At 1100, a query is sent to a flight controller and a timer is started.For example, recovery system controller 904 in FIG. 9A periodicallysends a query to flight controller 902. A timer in the flight controller(e.g., initially at 0) starts to count (e.g., upwards).

At 1102, it is determined whether a reply is received before the timerexceeds a timeout. For example, the timeout may be set to a relativelyhigh value that the flight computer, when operating normally, isexpected to easily and/or definitely respond by. If the timer exceedsthe timeout value without a reply being received, then that is anindication that the flight computer has failed (and therefore therecovery system controller should perform a recovery process). As such,if the timer exceeds the timeout without a reply being received at step1102, then a recovery process (e.g., FIG. 1) is initiated or otherwiseperformed at 1104.

Alternatively, if a reply is received before the timeout is exceeded atstep 1102, then at 1106 it is determined if the reply indicates that thevehicle is airworthy. For example, the flight computer may be responsiveand operational but may also have detected a failure in one of thevehicle's components (e.g., a motor controller is unresponsive, acontrol surface is stuck in a particular position, etc.). That detectedfailure and/or other indication that the vehicle is not airworthy may becommunicated in the reply.

If the reply indicates that the vehicle is not airworthy at step 1106,then a recovery process (e.g., FIG. 1) is initiated or otherwiseperformed at 1104. Otherwise, if the reply indicates that the vehicle isairworthy, it is decided whether to end the process at 1108. Forexample, the process may end when the vehicle lands or the vehicle ispowered down. If it is decided not to end the process at step 1108 thenat step 1110 the timer is reset (e.g., back to 0). The process repeatsat step 1100 when a next query is sent.

For brevity, this example shows the recovery system controllerinitiating a recovery process if either a timeout is exceeded or ifthere is an indication (e.g., from a flight controller) that the vehicleis not airworthy. Naturally, in some embodiments, a recovery systemcontroller may only check for one of these things. For example, if therewere no timeout check, then step 1100 would not necessarily start atimer and step 1100 would go directly to step 1106 (i.e., skipping step1102).

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, the invention is not limitedto the details provided. There are many alternative ways of implementingthe invention. The disclosed embodiments are illustrative and notrestrictive.

What is claimed is:
 1. A system, comprising: a recovery systemcontroller which is configured to: select between single stage parachutedeployment and multistage parachute deployment for a vehicle based atleast in part on vehicle state information; in the event multistageparachute deployment is selected: deploy a drogue parachute during afirst deployment stage; determine an amount of time to wait beforedeploying a main parachute in a second deployment stage based at leastin part on vehicle state information; and deploy the main parachuteduring a second deployment stage after the determined amount of time haselapsed; and in the event single stage parachute deployment is selected,deploy at least the main parachute in a single stage.
 2. The systemrecited in claim 1, wherein at least one of the main parachute or thedrogue parachute is deployed using one or more rockets.
 3. The systemrecited in claim 1, wherein selecting between single stage parachutedeployment and multistage parachute deployment is based at least in parton airspeed.
 4. The system recited in claim 1, wherein selecting betweensingle stage parachute deployment and multistage parachute deployment isbased at least in part on a tilt angle associated with a tiltrotorincluded in the vehicle.
 5. The system recited in claim 1, whereinselecting between single stage parachute deployment and multistageparachute deployment includes comparing the vehicle state informationagainst a threshold.
 6. The system recited in claim 1, wherein selectingbetween single stage parachute deployment and multistage parachutedeployment is based at least in part on a state variable associated withthe vehicle's mode of flight.
 7. The system recited in claim 1, wherein:in the event multistage parachute deployment is selected, the recoverysystem controller is further configured to determine an amount of timebetween deployment of the drogue parachute and deployment of the mainparachute based at least in part on the same vehicle state informationthat is used to select between single stage parachute deployment andmultistage parachute deployment; and the main parachute is deployedafter the drogue parachute has deployed and the determined amount oftime has elapsed.
 8. The system recited in claim 1, wherein: in theevent multistage parachute deployment is selected, the recovery systemcontroller is further configured to determine an amount of time betweendeployment of the drogue parachute and deployment of the main parachutebased at least in part on a second set of vehicle state information thatis different from the vehicle state information that is used to selectbetween single stage parachute deployment and multistage parachutedeployment; and the main parachute is deployed after the drogueparachute has deployed and the determined amount of time has elapsed. 9.The system recited in claim 1, wherein: in the event multistageparachute deployment is selected, the recovery system controller isfurther configured to determine an amount of time between deployment ofthe drogue parachute and deployment of the main parachute based at leastin part on a second set of vehicle state information that is differentfrom the vehicle state information that is used to select between singlestage parachute deployment and multistage parachute deployment; the mainparachute is deployed after the drogue parachute has deployed and thedetermined amount of time has elapsed; the vehicle state informationthat is used to select between single stage parachute deployment andmultistage parachute deployment includes airspeed and excludes vehiclevelocity direction; and the second set of vehicle state informationincludes airspeed and vehicle velocity direction.
 10. The system recitedin claim 1, wherein the recovery system controller is further configuredto: monitor three or more independent signals, each of whichindependently reports the vehicle state information; and in the event amajority of the independent signals agree, save the vehicle stateinformation from the majority of the independent signals that agree,wherein selecting between single stage parachute deployment andmultistage parachute deployment includes using a most recently savedvehicle state information.
 11. The system recited in claim 1, whereinthe recovery system controller is further configured to: send a query toa flight controller and start a timer; and in the event a reply is notreceived before the timer exceeds a timeout: select between single stageparachute deployment and multistage parachute deployment; in the eventmultistage parachute deployment is selected: deploy the drogueparachute; and after the drogue parachute has deployed, deploy the mainparachute; and in the event single stage parachute deployment isselected, deploy the main parachute without prior deployment of thedrogue parachute.
 12. The system recited in claim 1, wherein therecovery system controller is further configured to: send a query to aflight controller; and in the event a reply indicates that the vehicleis not airworthy: select between single stage parachute deployment andmultistage parachute deployment; in the event multistage parachutedeployment is selected: deploy the drogue parachute; and after thedrogue parachute has deployed, deploy the main parachute; and in theevent single stage parachute deployment is selected, deploy the mainparachute without prior deployment of the drogue parachute.
 13. Amethod, comprising: selecting between single stage parachute deploymentand multistage parachute deployment for a vehicle based at least in parton vehicle state information; in the event multistage parachutedeployment is selected: deploying a drogue parachute during a firstdeployment stage; determining an amount of time to wait before deployinga main parachute in a second deployment stage based at least in part onvehicle state information; deploying the main parachute during a seconddeployment stage after the determined amount of time has elapsed; and inthe event single stage parachute deployment is selected, deploying atleast the main parachute in a single stage.
 14. The method recited inclaim 13, wherein at least one of the main parachute or the drogueparachute is deployed using one or more rockets.
 15. The method recitedin claim 13, wherein selecting between single stage parachute deploymentand multistage parachute deployment is based at least in part onairspeed.
 16. The method recited in claim 13, wherein selecting betweensingle stage parachute deployment and multistage parachute deployment isbased at least in part on a tilt angle associated with a tiltrotorincluded in the vehicle.
 17. The method recited in claim 13, whereinselecting between single stage parachute deployment and multistageparachute deployment includes comparing the vehicle state informationagainst a threshold.
 18. The method recited in claim 13, whereinselecting between single stage parachute deployment and multistageparachute deployment is based at least in part on a state variableassociated with the vehicle's mode of flight.
 19. The method recited inclaim 13, wherein: in the event multistage parachute deployment isselected, determining an amount of time between deployment of the drogueparachute and deployment of the main parachute based at least in part onthe same vehicle state information that is used to select between singlestage parachute deployment and multistage parachute deployment; and themain parachute is deployed after the drogue parachute has deployed andthe determined amount of time has elapsed.
 20. A computer programproduct, the computer program product being embodied in a non-transitorycomputer readable storage medium and comprising computer instructionsfor: selecting between single stage parachute deployment and multistageparachute deployment for a vehicle based at least in part on vehiclestate information; in the event multistage parachute deployment isselected: deploying a drogue parachute during a first deployment stage;determining an amount of time to wait before deploying a main parachutein a second deployment stage based at least in part on vehicle stateinformation; deploying the main parachute during a second deploymentstage after the determined amount of time has elapsed; and in the eventsingle stage parachute deployment is selected, deploying at least themain parachute in a single stage.